THE AGE OF LONGCHUAN RIVER, AN IRRAWADDY TRIBUTARY

Longchuan River is an Irrawaddy tributary, one of the drainage systems originated in Tibet Plateau. It flows through the Tengchong basin where later Pliocene to later Pleistocene volcanics occur(Li et al., 2000; Mu et al., 1987). To reveal its age here we present regional geomorphological data and new ⁴⁰Ar-³⁹Ar ages of the volcanics, which show that Longchuan River is younger than 150 ka. The river passes through the “Shen-Zhu Gu”, God-column valley with a deep U shape. The ⁴⁰Ar-³⁹Ar age we obtained for its lower columnar basalt is 148.2±3.0 ka and that for the top layer is 147.7±2.9 ka. In Mangbang town through which Longchuan River flows the Pliocene Mangbang Formation consists of lacustrine deposits with diatomite and peat layers intercalated basalt lava, which yielded ⁴⁰Ar-³⁹Ar ages at 2.82±0.02 Ma and 2.79±0.01 Ma, which confirms previous K-Ar age results in the area (Li et al., 2000; Mu et al., 1987). The lacustrine deposits with diatomite layers but the lack of fluvial sediment show isolated lacustrine environments. All the evidence we obtained demonstrate the river’s geomorphology formed not earlier than later Pliocene, ca. 2.8 Ma. The striking young age of Longchuan River suggest that its drainage system evolves together with regional tectonic changes dramatically since that time. It occurred in the middle Pliocene when intense deformation in the Namche Barwa Syntaxis started, which resulted from the Australian Plate collided to SE Asia in early Pliocene and recorded in series of tectonic, sedimentary, Palaeoclimatology, Palaeoecology and Palaeogeography events (Curray, 2014; Deng et al., 2014; Kyi Khin et al., 2014; Seward & Burg, 2008; Zhang et al., 2012).

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